1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer and the like, and more particularly, it relates to an image forming apparatus having a charging means for charging a surface of an image bearing body.
2. Related Background Art
As image forming apparatuses of this kind, there are electrophotographic copying machines and electrophotographic printer, for example. In such apparatuses, an electrostatic latent image is formed by illuminating image exposure light corresponding to an original image or an inputted multi-value image signals onto a surface of an image bearing body uniformly charged by a charging means, and the electrostatic latent image is developed by a developing means to form a toner image. Then, the toner image is transferred onto a transfer material by a transfer means, and then, the transfer material is conveyed to a fixing means, where the toner image is fixed to the transfer material, and then, the transfer material is outputted as an imaged product (print, copy).
As an example of a charging device used with the above-mentioned image forming apparatus, there is a charging device of contact charging type as shown in FIG. 9. A charge roller (charging member) 11 is obtained by surrounding a metallic core 19 by a semi-conductive elastic member 20. The charge roller 11 is urged against an image bearing body 10 with predetermined pressure by means of a pressurizing means (not shown) and is rotatingly driven in a direction shown by the arrow L by rotation of the image bearing body 10 in a direction shown by the arrow K.
In synchronous with image formation, when charging bias voltage including AC component and DC component is applied from a charging bias voltage source 30 to the core 19 of the charge roller 11, the surface of the image bearing body 10 is charged with potential substantially equal to the DC component of the charging bias voltage. Hereinafter, the charging system of this kind is referred to as "AC bias contact charging type".
However, the charging device of AC bias contact charging type has a disadvantage that charge unevenness corresponding to frequency of the AC component of the charging bias voltage is generated.
FIG. 10 shows an example of the charge unevenness as distribution of electrification (charging) potential. In FIG. 10, the abscissa indicates a position on the surface of the image bearing body 10 along a rotational direction of the image bearing body 10, and the ordinate indicates charging potential on the surface of the image bearing body 10. The period of the charge unevenness corresponds to the frequency of the AC component of the charging bias voltage. The charge unevenness results in unevenness in density of the toner image after developing and, accordingly, unevenness in density of the outputted image, thereby worsening the image quality considerably.
It is known that, as the frequency of the AC component of the charging bias voltage is increased, such charge unevenness is gradually decreased.
On the other hand, the charging device of AC bias contact charging type has a disadvantage that, if the frequency of the AC component of the charging bias voltage is increased, the damage on the surface of the image bearing body is increased.
In the charging device of AC bias contact charging type, when the charging bias voltage including the AC component is applied to the charging member contacted with the surface of the image bearing body, a very strong electric field having alternating polarity is generated near a pole of the surface of the image bearing body. The alternating electric field generates a large amount of plasma ions and accelerates the ions. As a result, there arises a phenomena that, when the large amount of accelerated plasma ions strike against the surface of the image bearing body, the surface of the image bearing body is scraped. (This phenomena is referred to as "scraping phenomenon of surface of image bearing body due to contact charging" hereinafter).
That is to say, when the frequency of the AC component of the charging bias voltage is increased, since the number of ion collisions against the surface of the image bearing body is increased, the scraping phenomenon of surface of image bearing body due to contact charging becomes more noticeable.
If the scraping of the surface of the image bearing body is continued to make the photosensitive layer and insulation layer (of the surface of the image bearing body) thinner, the following disadvantages arise.
Firstly, if the photosensitive layer and insulation layer (of the surface of the image bearing body) are made thinner, the surface of the image bearing body is apt to be damaged. Secondly, as the electrostatic capacity of the surface of the image bearing body is increased, the charge amount required for obtaining the predetermined charging potential is also increased, with the result that the adequate charging potential cannot be obtained. And, the charging bias voltage source must made bulky to compensate such poor charging.
In conclusion, the conventional charging device of AC bias contact charging type arose problems that, if the frequency of the AC component of the charging bias voltage is small, the charge unevenness is generated and that, if the frequency of the AC component of the charging bias voltage is increased, the service life of the image bearing body is shortened and the charging bias voltage source is made bulky to made the apparatus more expensive.
On the other hand, there has been proposed an image forming apparatus in which a character mode (suitable for a character image) having an image forming condition corresponding larger inclination y and a photograph mode (suitable for photographic (intermediate gradation) image) having an image forming condition corresponding to smaller inclination y are provided (here, the "inclination" is defined as inclination in a graph showing property of density of the outputted image with respect to density of an image of an original or an image signal inputted from an external equipment). In the image forming apparatus having such character mode and photograph mode, when the above-mentioned charging member to which the voltage including the AC component is used, it was found that a bad influence upon the image due to the charge unevenness generated in correspondence to the frequency of the AC component of the charging bias voltage becomes noticeable particularly in an intermediate density portion of the image.
Japanese Patent Application Laid-open No. 5-11571 discloses a technique in which the frequency of the AC component is switched between a character pattern and a graphic pattern, but does not disclose a mode for changing the inclination .gamma..
USP 5512982 discloses a technique in which the frequency is switched between a character mode and a photograph mode, but does not disclose a technique in which the inclination .gamma. is changed in accordance with the modes.